(_h_) The claim made by the older generation of surgeons that shock could be prevented by the use of a _deep_ anæsthesia, and that the occurrence of any “Reflex syncope” was always a sign of too light an anæsthesia cannot be made good. At the same time, it must be admitted that too light an anæsthesia does increase the likelihood of shock. _Prolonged deep_ anæsthesia, on the other hand, produces by itself a condition indistinguishable from shock, with the single exception of nitrous oxide gas. (_i_) Operative shock is predisposed to by several factors of which the following are the most important:-- 1. Hæmorrhage before or during operation. 2. Sepsis. 3. Fear. 4. Prolonged starvation. 5. Certain diseases, especially hyperthyroidism (exopthalmic goître). Theories of Shock. So far as we have touched in the above upon theory, it has been theory which receives general acceptance and which accords with known clinical facts. When we come to discuss _the reason why blood-pressure falls in shock_, we are in more debatable country. Crile’s original view was that the upstroke seen in such charts as shown in Fig. 1 are caused by reflex vaso-constriction, and that the final fall of B.P. was due to exhaustion of the vaso-motor centre. This view he does not seem to have modified as a result of his later discovery of degenerative changes in certain cells of the grey matter of the brain. Other workers, J. D. Malcolm in this country, and Yandell Henderson, of Yale, U.S.A., maintain an opinion diametrically opposite. In their view, in fully developed shock, the vessels are in vaso-constriction, and the circulation is arrested from undue internal resistance to blood flow. A third explanation of lowered B.P. has been offered, and while its significance is not understood, there is fairly general agreement as to its validity. This factor is a _reduction in the total blood volume_--an oligæmia. No one has as yet demonstrated to what region or organ the missing blood volume has retreated. YANDELL HENDERSON’S ACAPNIC THEORY OF SHOCK. Acapnia is a condition in which the CO_{2} content of the blood and tissues has been brought to too low a level. Those who climb mountains suffer from it, and so do those who breathe rapidly and heavily for a prolonged period. Carbon dioxide is necessary for the vigour of the respiratory centre, of which it may be termed the natural regulator. Moreover, the heart and the great veins which empty into it require a certain proportion of CO_{2} in the blood.